This way it is easier for people to actually find the broken ones so we
can blacklist them explicitly.

io.flush() is needed, otherwise nothing gets printed until the loading
is finished.

This turned out to be easier that I first thought.

I didn't know how to check if a table contains a certain value so I had
this value=true hack, now I know better so removed it.

Blacklisted fonts are listed one per line in a file named
"otfl-blacklist.cnf", there can be as much copies of this file as kpse
can find (we look for "tex" format despite of the file extension, so it
can exist any where a .tex file can) and all found files will be loaded
an read.

The parser is rather dump, it assumes a good formated file; one font
per line, no trailing spaces, no inline comments, lines starting with
"%" and empty lines will be ignored, any other lines will end in the
blacklist table without any further processing.

This reverts commit 1473edf932ec729f4a41736fb7ee52654c848d59.

This broke mkluatexfontdab.